Chapter 8 - The Human Sexual Response PDF

C H A P T E R 8 The Human Sexual Response INTRODUCTION through our behavior (sexual or otherwise), speech,...

C H A P T E R 8 The Human Sexual Response INTRODUCTION through our behavior (sexual or otherwise), speech, dress, and so on. We are deemed feminine if our sex role Previous chapters described how the human egg and fits what society defines as the female role and masculine sperm are formed and become mature. These gametes if the form of our sex role fits society’s definition of what are then brought together during the sexual response is male. The term androgyny refers to one having both cycle, which involves not only the mechanics of coitus masculine and feminine characteristics. Our sex role is (sexual intercourse), but also sexual arousal and other influenced greatly by our culture, social interactions, the behaviors surrounding sexuality. This chapter discusses nature of our home and family, and religious beliefs, as the response cycle in men and women and shows how well as biology. malfunction of the cycle can occur. We also discuss gen- der identity and sex roles, and how hormones, phero- Nature or Nurture mones, and drugs can influence our sex drive and the Controversy has existed over whether our sex role is sexual response cycle. mainly a product of our biology (our “nature”) or of our learning and experience (our “nurture”). Are we femi- nine or masculine because of our biological makeup or because of our rearing and present environment? In the SEX ROLES past, most favored the theory that nurture plays a more important role. Anthropological studies had shown Factors Influencing Sex Roles that sex role is highly influenced by social training and Sex roles are a product of our biological nature, how culture. we perceive this nature, and how we present our sexu- A modern theory of gender identity and sex role ality to others. First, our biological sex determines our describes a continuous interaction of biological and anatomical and physiological femaleness or maleness. If social influences from conception. This theory holds our cells have two X chromosomes we develop ovaries, that chromosomal and hormonal makeup determines female sex accessory structures, and female secondary whether the genitals (and perhaps the brain) develop in sexual characteristics (see Chapter 5). If our cells have a male or female direction. Sex differentiation, a biologi- one X and one Y chromosome, we develop testes, male cally determined event (see Chapter 5), is followed by the sex accessory structures, and male secondary sexual increasing influence of psychological and social input. characteristics. In addition to these anatomical and How individuals perceive their bodies (i.e. whether physiological sex differences, our biological maleness they have male or female sex structures) leads to a body and femaleness also include sex differences in brain image of maleness or femaleness (gender identity). Gen- function that could influence our sexual behavior (see erally, this awareness of one’s maleness or femaleness Chapter 16). begins around 18 months of age. The expression of this gender identity through sex roles develops later and is Gender Identity influenced greatly by familial and social factors. That One’s gender identity is the psychological belief or is, one’s sexual behavior, appearance, and even the way awareness that one is biologically female or male. It is one thinks are all product of training. the private experience of one’s biological sex. Sex role (or Children tend to imitate the parent of the same sex, gender role) is the outward expression of gender identity. and parents and peers behave differently toward boys It is the way we present our gender identity publicly and girls, reinforcing or punishing certain behaviors. Human Reproductive Biology, Fourth Edition. 135 Copyright © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/B978-0-12-382184-3.00008-8 136 8. THE HUMAN SEXUAL RESPONSE Although there are sex differences in behavior at birth erotic; the voice of a sexual partner or soft music can set (Chapter 16), any role of these differences in the develop- the scene for sexual interaction. The taste of certain food ment of sex role is conjectural at this time. Thus, sex role or drink may be associated with past sexual encounters is the result of some undefined interaction between bio- and can be sexually arousing. Although humans are not logical sex, environmental input, and a person’s aware- considered to rely on smell as much as other mammals, ness of self and their relationship to others. This theory is smells can be associated with past sexual encounters still undergoing revision, and much remains to be learned and can be arousing, as evidenced by the commercial about this fascinating and important subject. For exam- sales of scents, perfumes, and colognes. Also, certain ple, transsexuals (people with a gender identity opposite chemical signals exuded by our bodies may play a role in from their biological sex) have been shown to have a part our sexual biology (see Box 8.1). of the brain typical of the opposite sex (see Chapter 16). Erogenous Zones SEXUAL AROUSAL Touch (or tactile) stimuli are important for sexual arousal in both sexes. The body is particularly sensitive sexually in One is sexually aroused when environmental factors certain regions; these are known as the erogenous zones. In or internal thoughts initiate a sexual response. Erotic males, examples are the glans, corona, and lower side of stimuli are those factors in the environment that are sexu- the penis. In females, examples include the clitoris, mons, ally arousing to an individual. A detailed survey of the labia minora, and lower third of the vagina. The upper kinds of erotic stimuli in our species would be impossible two-thirds of the vaginal wall are relatively insensitive to because what is erotic differs greatly among individuals touch. Erogenous zones in both sexes include the nipples, and is highly subject to past experience. What excites lips, tongue, ear lobes, anus, buttocks, inner thighs, and one person may not be sexually arousing to another. even the back of the knees, soles of the feet, and center of the back. There is, of course, individual variation in the sensitivity of these areas. Cultural Influence and Individual Variation There is certainly a cultural influence on what is per- Proceptive Behavior ceived as erotic. In the United States, the stereotypes of the sexually attractive woman and man are perpetuated Proceptive behavior is the scientific term describ- by the media, advertisements, and movies. But what may ing courtship, flirting, seduction, and even foreplay in be true in our culture may not be true in other cultures. humans. Although human proceptive behavior is influ- For example, many American men are sexually aroused enced by culture and tradition, it also has some features by the sight of female breasts, but in cultures where the that appear to be universal in all human cultures and women do not commonly wear clothing on their upper therefore could have evolved as patterns in our ances- bodies, this may not be so. Also, in our culture, being tors (just as may also be true for human sexual attrac- thin is considered sexier than being heavy, but this is tion and mating strategies). These cross-cultural features not true in some other cultures. Nevertheless, there also include eye contact, a slight smile, talking about superfi- appear to be universal (cross-cultural) common pat- cial things with a high degree of animation, rotating the terns of what is sexually attractive, such as a youthful face toward each other, moving closer together, moisten- appearance in females and symmetrical facial features ing the lips, and perhaps revealing or emphasizing parts in both sexes. Within a given culture, there can be great of the body. In addition, a courting couple can “acciden- individual variation in what is perceived as erotic. One tally” brush together or touch, as well as mirror and syn- woman may be aroused by a man’s open shirt and hairy chronize with the other’s postures and movements. chest, whereas another may especially like men’s fore- Kissing the lips can, of course, merely be a sign of arms. One man may be excited by the sight of a woman’s affection and not have sexual overtones. Kissing, how- ankles, whereas another is stimulated by long hair. ever, is a common form of sexual arousal in the United States and other countries. This can be done simply by a couple pressing their lips together with gentle move- Erotic Stimuli ment. In “French kissing,” the tongue of one or both Erotic stimuli can be mental, originating in erotic partners is inserted and moved within the other’s oral thoughts, or perceived by any of our senses: vision, hear- cavity. In other cultures, kissing is not a usual form of ing, smell, touch, or taste. Visual images are primary erotic sexual arousal. People in Japan, China, and Polynesia, stimuli to many individuals. Tactile stimuli may lead to for example, did not kiss until they had contact with sexual arousal, especially if associated with erogenous Westerners. In some cultures, such as the Thonga of zones (see the following discussion). Sound can also be Africa, kissing the lips is looked upon as repulsive. III. PROCREATION  Sexual Arousal 137 BOX 8.1 HUMAN PHEROMONES AND THE VOMERONASAL ORGAN The predominant human sexual senses are often consid- males when they are from women in the follicular phase ered to be vision, touch, and hearing. To other mammals, of the menstrual cycle (i.e. before ovulation) than in the however, the sense of smell plays a critical role in sexual luteal phase, suggesting that men may be able to sense interactions. When a member of a species emits a chemical when a woman is fertile by her pheromones. that changes the physiology or behavior of another mem- We already discussed (Chapter 3) evidence that women ber of the same species, this chemical is called a pheromone. housed together tend to have menstrual cycles that are For example, there is a substance in the urine of adult male synchronized, and a recent study suggests that an odor mice that accelerates the onset of puberty in female mice produced by the armpits (axilla) of women may cause this and also synchronizes the estrous cycles of adult females. A synchrony. Other studies indicate that the frequency of male rat can tell when a female is in estrus by a pheromone exposure to men can influence menstrual regularity and in her urine. Similarly, a female Rhesus monkey that is ovu- length, and some evidence suggests that this male effect is lating exudes pheromones (“copulins”) that increase sex caused by an axillary pheromone. drive in male monkeys. Vaginal secretions of female ham- In most mammals, pheromones are sensed by special sters contain “aphrodisin,” a chemical that attracts males. sensory organs in the floor of the right and left internal The saliva of male pigs contains an androgen that makes nasal cavities, separate from the olfactory cells on the roof sows sexually receptive. of the nasal cavity that we use to smell nonsexual odors. We are just beginning to investigate the idea that pher- These special pheromone sense organs, the vomeronasal or- omones influence human behavior, a possibility that the gans (VNO), are present in the human fetus during the first manufacturers of perfumes and colognes have assumed trimester, after which they degenerate. Recent research has for years. Some artificial, musk-like odors can be smelled revealed that many adult humans have a small, vestigial by adult women and not by children or males. What is VNO on the floor of the nasal cavity on each side of the na- more, women are maximally sensitive to this odor near sal septum (the wall dividing right and left internal nasal the time of ovulation. Is it possible that men produce a cavities). Each VNO opens into the nasal cavity via a tiny musky pheromone that influences female sexual behavior pit. However, the adult human VNO lacks functional neu- and that women differ in their ability to sense this smell in ronal connections that could transmit pheromonal signals accordance with the stages of the menstrual cycle? There to the brain. Does the absence of a functional VNO mean are glands on the male skin, axilla, penis, and scrotum that that there are no pheromones in humans? Recent research could produce pheromones. has overturned the notion that the VMO is the exclusive re- Androstanes, steroids related to androgens, are present ceiver of pheromonal signals in vertebrates, and that some in the axillary (underarm) secretions of men in concentra- pheromones are actually detected by the regular olfactory tions up to 20 times greater than in females. Inhalation of epithelium. Thus, it is possible that human pheromonal these molecules causes activation of the ventroanterior cues could be received and transmitted to the brain by the hypothalamus in women, but not in men. In experiments olfactory epithelium that also serves our sense of smell. where these molecules were placed under women’s noses, Recent research has shown that women are attracted the women demonstrated enhanced positive mood and to the scents of men who are most unlike themselves in alertness, but only in the presence of a male. Further- major histocompatibility complex genes (MHCs). These genes more, women exposed to androstanes judged men to be encode cell surface proteins that offer a “display” of pro- more attractive than did women who did not inhale the teins being made inside the cells. Thus the immune system molecules. can detect foreign proteins (such as those from a virus or Women may also produce pheromones. The vaginal se- toxin) in a cell and destroy the cell accordingly. The more cretions of women, especially when they are near the time dissimilar the mate’s MHCs, the better their offspring’s of ovulation, contain several chemicals that are known to immune system will be at detecting foreign proteins. In be copulins in monkeys. We do not know of any behav- fact, human mates with similar MHCs tend to be less fer- ioral effects of these potential pheromones on men. How- tile and have more miscarriages. Therefore, it could be an ever, evidence indicates that vaginal secretions applied to evolved, adaptive trait that humans can “smell” different the bodies of women increase their sexual attractiveness. MHCs (or their associated chemical cues) in a potential Is it possible that feminine hygiene sprays are covering mate and avoid similarity. The possible roles of phero- up something biologically meaningful? Another study mones in human sexual behavior continue to be a fasci- showed that female body odors smell more pleasant to nating and controversial topic. (Continued) III. PROCREATION 138 8. THE HUMAN SEXUAL RESPONSE BOX 8.1 (cont’d) Olfactory epithelium Nasal septum Vomeronasal organ    The anatomical location of the vestigial human vomeronasal organ (VNO). Note that the paired VNOs are located about a half inch into the nasal cavity, on its floor on each side of the nasal septum (the midline partition that divides the nasal cavities into right and left). When present, each VNO is a sac with an opening about 1 mm in diameter. Note also that the olfactory epithelium is on the roof, not the floor, of the nasal cavities on each side. Mild pain also can be an erotic stimulus. Gentle nib- THE SEXUAL RESPONSE CYCLE bling, biting, pinching, and scratching can be sexually arousing for some couples. In some cultures, minor pain In the not-too-distant past, the human sexual response is often associated with arousing interactions. For exam- was little understood and was not the subject of much sci- ple, the Apinaye females of South America bite the part- entific observation. Alfred Kinsey opened the door to the ner’s eyebrow as part of the sexual encounter, and the scientific study of human sexuality, but it was not until the Trukese women of the South Pacific poke a finger in the studies of William Masters and Virginia Johnson that we ear of their partner. In our culture, forms of sexual inter- began to know more about the physiology of the human action that cause intense pain are considered deviant. sexual response. Although their methods have been criti- The partners in a sexual interplay are responsible for cized by some, their research has had a profound effect on giving effective erotic stimuli to each other in the process of the understanding of ourselves, on research in human sex- foreplay, or “petting.” To be effective, foreplay requires open uality, and on the treatment of sexual dysfunction. Most expression of emotions, communication, and consideration of what is said here about the sexual response cycle will, of place, pace, and style. During foreplay, several means of therefore, be based on the studies of Masters and Johnson. arousing one’s partner can be utilized, some more common In both sexes, the sexual response cycle can be divided than others. For many young people, especially those wish- into four phases: excitement, plateau, orgasm, and resolu- ing to avoid coitus, foreplay is the “only play.” For many tion. Figure 8.1-8.3 depict these phases in the female and other couples, however, foreplay is a prelude to coitus. In male. Although these are described as distinct phases, one study, foreplay of married couples for 1–10 min led to they effectively flow into one another as a continuous 40% of the wives having orgasm during coitus. This per- cycle if effective erotic stimuli are present. If these stim- centage rose to 50% with 12–20 min of foreplay and to 60% uli are not adequate, however, the initial phases are not with greater than 20 min of foreplay. It is, of course, not followed by the final phases. Our discussion focuses on only the time spent in foreplay that is important. It is the the sexual response cycle during heterosexual coitus; quality of the stimuli, including a good dose of love and however, the full cycle in either sex can occur during affection, that is especially satisfying and arousing. masturbation or homosexual sex as well. III. PROCREATION  The Sexual Response Cycle 139 Bladder Uterus Uterus elevates Rectum Clitoris Vagina enlarges expands Vaginal (”tenting”) lubrication appears Color Orgasmic Labia swell platform change in labia Excitement Plateau Contractions in uterus Uterus lowers Seminal pool Rhythmic Vagina returns contractions to normal in orgasmic platform Anal sphincter Orgasmic contracts platform disappears Orgasm Resolution FIGURE 8.1 The female sexual response cycle. 1. T he inner two-thirds of the vaginal barrel begin The Female Sexual Response Cycle to increase in length and width. Thus, the vaginal The female sexual response cycle is similar to that of cavity, which is closed at rest, begins to widen. the male in that it is divided into four phases. As we see, 2. The body of the uterus ascends (the tenting effect), however, there are some important differences between pulling the cervix away from the vagina and thus male and female cycles (see Figure 8.2). further increasing vaginal length. There can also be rapid, irregular uterine contractions (fibrillation). Excitement Phase These uterine contractions are not painful. The size The female excitement phase (Figure 8.1) is initi- of the uterus also increases due to vasocongestion ated by the presence of effective erotic stimuli. The (pooling of blood in blood vessels). first change, usually occurring within 10–30 s, is vaginal 3. The walls of the vagina become engorged with blood lubrication, i.e. the membrane lining the vagina becomes and become darker in color. more moist. It used to be thought that this was caused 4. The shaft of the clitoris increases in diameter by secretions from the Bartholin’s glands, but the work (but rarely in length), and there may be a slight of Masters and Johnson showed this not to be the case. tumescence (swelling) of the clitoral glans due to Instead, the fluid leaks out of blood vessels present in vasocongestion. the vaginal wall. Other responses occurring during the 5. The labia minora become engorged with blood, and female excitement phase include the following: their size increases considerably.    III. PROCREATION 140 8. THE HUMAN SEXUAL RESPONSE BOX 8.2 H U M A N M AT I N G S Y S T E M S Animal mating systems can be of several types. involved with child care, and this system fosters more Monogamy is when a single female and male pair within equality between the sexes. Monogamy may reduce the one breeding effort or for a lifetime. Polygamy occurs when birth rate as females must rely on a single male (who may an individual of one sex pairs simultaneously with more have low fertility) for sperm to fertilize her eggs. than one member of the other sex. Within polygamy, Polygyny (e.g. a dominant male with a harem of several polyandry is when one female pairs with more than one females) is more adaptive when environmental resources male, and polygyny is when one male pairs with more than (e.g. water, food) are patchy (clumped) and when females one female. Promiscuity is when no lasting pair-bonds are occur in groups. Thus, males compete for acquisition of a established and partners change frequently. group of females by fighting with other males and display- To understand mating systems, we must consider that ing their prowess to females. Through a process Charles the ultimate “goal” of both males and females is to maximize Darwin called sexual selection, this leads to sexual dimor- their evolutionary fitness (i.e. number of healthy offspring). phism (e.g. large male body size and “ornaments” of the To achieve reproductive success, a mammalian female needs male for display). The ovarian cycles of individual females ready access to a mate when she is ovulating. Furthermore, in polygynous species tend not to be synchronized because she must be well fed and protected during her lengthy and if they were it would be difficult for a male with a harem metabolically expensive pregnancy and lactation, and have to mate with all of his females at once! Furthermore, the help raising her young. In contrast to the extensive time and males of polygynous species participate less in parental energetic input required by a woman to raise a child, men care, and the young tend to be born in a less helpless state. can successfully pass their genes to the next generation in Among primates, the group to which humans belong, a single mating episode. Sperm are cheap and abundant, so 15% are monogamous, a higher percentage than for mam- the male strategy is to impregnate as many fertile females as mals in general. Among the great apes, our closest living possible. Thus, polygyny allows males to leave a large num- primate relatives, there is an interesting variety of mating ber of offspring, but it comes at a price—only those males systems. The gorilla, for example, is polygynous. A large who are large, strong, powerful, or rich enough to compete male acquires a harem, and he then mates periodically against other males and attract many females are successful when a sexually receptive female in his harem presents at this strategy. Because of the investment a female puts into herself to him. Gorillas are highly sexually dimorphic, with each pregnancy, she is likely to be more choosy about the the males being twice the size of the females. Surprisingly, quality of her mate, selecting for “good genetics” and pos- the male gorilla’s testes and penis are relatively small; there sibly a male who will help care for her offspring. is no need to produce a lot of sperm because mating is in- Monogamous species tend to be sexually monomorphic frequent and the male knows that any pregnancies in his (the sexes look alike), have offspring that require extended harem are his; there is no need to stimulate females or dis- parental care from both parents, and evolve in environ- play (with a large penis) to females he already possesses in ments in which resources (e.g. food) are spread out and his harem. The chimpanzee, however, has a promiscuous predation is high. Also, individuals in the population tend mating system within a social group. Mating is frequent, to be widely scattered across the landscape. The idea is and partners switch regularly. The sexes are about the that the male stays with one female to ensure viability of same size (monomorphic), but the testes of a male chimp are their offspring by helping care for the young, defending relatively large (sperm for lots of matings and to compete their young from other males (who could kill them) and with other males’ sperm). The penis of a chimp is also rela- predators, and obtaining food. Finally, monogamous pairs tively large; it is used to display to the females. The female tend to exhibit more “affectionate” behavior to maintain chimps advertise their receptivity by a swelling in the peri- the pair-bond. Monogamy is common in some animal neal region. The whole social group helps the mother care groups; e.g. 92% of birds are monogamous for at least one for the young, gather food, and defend against predators. mating season. However, only 3% of mammalian species What about humans? There is a moderate size dimor- are monogamous, with the great majority being polygy- phism between men and women: men are on average nous. In humans, a monogamous system may benefit so- about 10% larger. Patterns of muscle development, fat de- ciety because it reduces male–male competition that leads position, and hair growth also differ between males and to violence and murder. When more men are married, females. This dimorphism suggests an evolutionary pat- they are less likely to be involved in risk-taking behaviors tern with some degree of polygyny. However, humans also such as crime and substance abuse. Males may be more show characteristics consistent with those of monogamy. III. PROCREATION  The Sexual Response Cycle 141 BOX 8.2 (cont’d) In men, the testes are relatively small (no need for a lot of sperm). However, the penis is relatively large (either for dis- play or for giving sexual pleasure). The human female ex- hibits several unique traits among primates, and these traits may have evolved to promote a prolonged pair-bond (to keep the male from wandering). Human breasts, unlike oth- er primates’, remain enlarged even when not nursing and serve as a sexual attractant. The breasts of other primates are not enlarged except when producing milk. Also, humans are the only primate to have coitus face to face, which al- lows more eye contact and affectionate interaction. Further- more, human females have concealed ovulation, i.e. a male of our ancestors was unlikely to be able to tell when his mate was ovulating because she appeared in “estrus” at all times. Therefore, our ancestral male may have had to stay with his mate and copulate periodically to ensure that her pregnancy was his. Finally, it has been argued that because the human newborn is born in a relatively helpless state needing ex- tended parental care, this favors a monogamous mating sys- tem. However, monogamy is not a prerequisite for paren- tal, even paternal care, and a close social or kinship group of women is the model for child rearing in many cultures. Thus, the need to share responsibilities to provide extended care for young may have shaped human social evolution at least as much as it has influenced mating systems. A survey of 853 present-day human cultures, from de- veloped to hunter-gatherer, has shown that 84% of human cultures permit polygyny, although only 10% of men in these cultures actually have more than one wife. This is probably because few men have the social status or re- sources to care for multiple wives and their children, and thus most men, even in polygynous cultures, practice mo- nogamy. Thus, monogamy appears to be the main present- day human sexual strategy, with polygyny being a second- ary choice. Humans also have a tendency toward serial monogamy, i.e. changing monogamous partners when their children become more independent. Serial monog- amy has been shown to increase the evolutionary fitness of men (i.e. divorced and remarried men father more chil-    dren than men married for life) but does not benefit wom- An artist’s rendition of a seemingly affectionate pair of en in this way. In a recent survey in the United States, 83% Australopithecus afarensis (of “Lucy” fame) strolling in the hot of men or women said that they were monogamous (mar- sun of the African savannah. This hominid primate may have ried or cohabiting) and had only one or no sexual partners been an African ancestor of Homo habilis, the first member in the past year. However, the same survey reported that of the genus of humans. Males were quite a bit larger than 25% of married men and 15% of married women report- females, an indication of a polygynous mating system charac- ed that they were unfaithful to their mates. On average, terized by male–male competition. However, the size dimor- women across the world’s cultures are less promiscuous phism of canine teeth one would expect to be associated with than men. Similarly, polyandry is rare in human cultures, male competition is missing. Because of this unusual combina- possibly because women have limits to their reproductive tion of traits, researchers are not agreed on the mating system potential, and additional matings would not increase their of this hominid. Source: from the American Museum of Natural fitness if they already have high-quality mates. However, History http://images.library.amnh.org/digital/items/show/4609. (Continued) III. PROCREATION 142 8. THE HUMAN SEXUAL RESPONSE BOX 8.2 (cont’d) female fitness is benefited by seeking another mate if the (­especially men) began to own animals, parcels of land, ­current mate is unable to provide healthy sperm for a suc- and crops. Their possessions extended to wives, whom cessful pregnancy. Extra-pair copulations can benefit the they could house on their own property and thus moni- ­fitness of both males and females, but may have too great tor closely. Successful individuals accumulated wealth, a social cost (especially for females) in some cultures. wives, and children. Families began to gain power, con- It is likely that as humans evolved, so did their mat- solidating wealth in lineages. In this polygynous social ing systems. One theory suggests that for about 95% of system, perhaps only prosperous men were able to keep human evolutionary history, we were hunter-gatherers in multiple wives. Whether monogamy prevailed among small, fairly egalitarian groups in which men and women those less well off remains an unanswered question. shared food, shelter, daily tasks and responsibilities, child Thus, although monogamous marriage is considered rearing—and sexual partners. In this relatively promiscu- the norm in the United States today, throughout human ous scenario, child rearing was shared by females in the evolution different cultures may have employed, and con- group, and, because paternity was difficult to track, it tinue to use, a variety of mating systems. Perhaps this is benefited males to guard and provide for children in the why modern marriage involves elaborate ceremonies with social group, as they might be their own. With the “vows” and “commitments” that would be unnecessary if ­advent of agriculture about 10,000 years ago, individuals humans were hardwired for lifelong monogamy. 6. T he labia majora, which at rest lie over the vestibule, 5. T he sex flush, if present, spreads and becomes more flatten out and retract from the midline. intense. 7. The nipples become erect, the areola becomes wide 6. Heart rate, blood pressure, and the depth and rate of and darker, and the size of the breasts increases breathing increase. slightly due to fluid accumulation. 7. There is a further increase in muscular tension.    8. A sex flush begins to appear in about 74% of women, i.e. areas of skin become reddened due to dilation of Orgasm Phase blood vessels. It usually begins on the abdomen and throat and then spreads to the chest, face, and even As discussed later in this chapter, not all women the shoulders, arms, and thighs. have an orgasm all the time. The female orgasm phase, 9. There is an overall increase in tension in voluntary if stimulated by coitus, usually occurs 10–20 min after and involuntary muscles (myotonia). intromission (penetration of the penis into the vagina).    The word orgasm (“climax”) comes from the Greek word orgasmos, which means “to swell” or “be lustful.” Plateau Phase An orgasm in either sex can be one of the most intense During the female plateau phase (Figure 8.1), the fol- and pleasurable of human experiences. We talk more lowing changes occur if effective erotic stimuli are present: about the experience of orgasm later, but now let’s look    1. T he wall of the outer one-third of the vagina at some physiological changes that occur during the becomes greatly engorged with blood so the vaginal orgasm phase (Figure 8.1):    cavity is reduced compared to the excitement phase. 1. S trong muscular contractions occur in the outer Also, the labia minora become more engorged with one-third of the vaginal wall. The first contraction blood and thus become redder and larger. These lasts about 2–4 s and is followed by rhythmic changes in the outer third of the vagina, and the labia contractions at intervals of 0.8 s, approximately minora, are called the orgasmic platform because the same frequency as the muscular contractions they indicate that orgasm is imminent. during male ejaculation. There can be 3–15 of these 2. The clitoris retracts to be completely covered by contractions, and the intensity of the initial ones is the clitoral hood, and its length decreases by about greater than that of later ones. The rectal sphincter 50%. Thus, from this stage on, the clitoris can be can also exhibit rhythmic contractions at 0.8 s stimulated directly only through the hood or mons intervals. and indirectly by tension applied to the labia minora. 2. The inner two-thirds of the vagina often expands, 3. Uterine fibrillation continues and may increase in which facilitates movement of the penis within it. intensity. The uterus also elevates further. 3. Rhythmic contractions of the uterus occur, 4. The nipples become more erect and the areola probably brought about by release of the hormone darker; the breasts reach their maximal size. oxytocin. III. PROCREATION  The Sexual Response Cycle 143 (A) Refractory Refractory vagina and lower pelvis. Other experiences, varying period period from one woman to another, include mild twitching of the extremities, body rigidity, facial grimacing, and utter- Orgasm ing of groans, screams, laughter, or crying. Some women even report that they lose consciousness briefly. Surveys Plateau of the experiences of orgasm in women and men sug- Res gest that the pleasurable feelings of both sexes during R es Excitement orgasm are similar. In fact, one study of positron emis- olu olut tion sion tomography scans of the brains of men and women ion during orgasm showed nearly identical activity in brain (B) areas associated with pleasure, which was similar to that Orgasm of a person under the influence of heroin! There may be several kinds of orgasm in women. One kind, the clito- ral orgasm, was described by Masters and Johnson. This Re so type results from stimulation of the clitoris during coitus Plateau lut ion or masturbation. Vaginal orgasms, however, are thought Resolutio to be the result of direct stimulation of the vaginal wall. Res There may be a small region in the front wall of the olut (b) vagina that, when stimulated, can produce sexual arousal ion Excitement n and orgasm. This region is termed the Grafenberg spot. a bc (c) (a) The orgasm that results from stimulation of this region FIGURE 8.2 The sexual response cycle in men (A) and women (B). involves intense contraction of the uterus and pubococ- Curves represent degrees of sexual arousal. The three different female cygeus muscle and has been called an A-frame orgasm or cycle patterns in (B) are discussed in the text. uterine orgasm. In reality, most orgasms probably involve a blend of the aforementioned kinds. A woman certainly should not suffer from “performance anxiety” if she does 4. T he sex flush, if present, peaks in intensity and or does not have a particular kind of orgasm. distribution. Does female ejaculation occur during orgasm? Recent 5. The heart rate, blood pressure, and depth and rate of studies indicate that about 10% of women expel a small breathing peak at rates similar to those during male amount of fluid into the vestibule during orgasm. This orgasm (see later). fluid actually comes from the lesser vestibular (Skene’s) 6. There may be strong involuntary muscle contractions glands near the urethral opening. These glands are homol- and clutching or clawing motions of the hands and feet.    ogous to the prostate gland of the male (see Chapter 5). There is also a release of neuromuscular tension. The It is not true that the size of a man’s penis bears a conditions of the labia minora, labia majora, clitoris, and direct relationship to sexual satisfaction in the female. breasts remain similar to those in the plateau phase. This is because the vagina adapts in size. However, an One major difference between the female and the male extremely small penis may not provide sufficient stim- sexual response cycle (see Figure 8.2) is that the female does ulation, whereas an extremely large penis may cause not have a refractory period right after orgasm, which the discomfort. It should also be mentioned that there is no male has, as discussed later. Kinsey first reported that only benefit to simultaneous orgasm in a man and woman about 14% of women in his study had multiple orgasms if unless this is an achievable and pleasurable goal of a cou- effective stimuli were present, but probably more women ple’s sex life. In fact, if a woman enjoys multiple orgasms, are physically capable of this experience. Such women it may be necessary that the man delay his orgasm. report that later orgasms in the sequence are more intense It is sometimes the case that the vagina becomes so than the initial one. A few women can have status orgas- relaxed that it leads to less sexual stimulation during coitus. mus, which is a sustained orgasm lasting up to a minute. This is a common complaint of women who have had sev- The experience of orgasm can vary in one woman and eral children. In this case, the couple can try new coital among different women. In studies of female sexuality positions, and the woman can exercise the pubococcygeus in the 1970s, Shere Hite reported that this experience muscle (see Chapter 2) to strengthen the vaginal wall. often occurs in three stages. First, women experience a sensation of “suspension,” lasting only an instant, fol- Resolution Phase lowed by a feeling of intense sensual awareness, oriented After orgasm, and if there are no effective erotic stim- at the clitoris and radiating upward into the pelvis. In uli present, the woman’s system returns to normal dur- the second stage, there is a sensation of warmth, begin- ing the resolution phase (Figure 8.1). Some symptoms ning in the pelvis and spreading to other parts of the return to normal rapidly. In less than 10 s after orgasm, body. Finally, there is pelvic throbbing, focusing in the vaginal contractions cease and the clitoris leaves its III. PROCREATION 144 8. THE HUMAN SEXUAL RESPONSE retracted position. The heart rate, blood pressure, and reaching orgasm. This pattern often occurs in inexpe- respiration decline quickly to resting levels. Also, the rienced women or if inadequate stimuli are present. In labia minora return to their original color, usually within pattern C, stimuli produce an early, intense orgasm. 2 min. The internal cervical os dilates immediately after There is considerable variation in the ability to have orgasm, perhaps to allow sperm to move into the uterus an orgasm. About one in three women seldom or never (see Chapter 9). Muscle tension decreases in about 5 min, reach an orgasm during coitus. Although many of these and the breasts decrease in size in 5–10 min. Vasocon- women are able to experience orgasm using other kinds gestion in the clitoris, vagina, and labia minora ebbs in of sexual stimulation, one in five women reports rarely 5–10 min, and the uterus usually returns to its normal or never achieving climax during masturbation. Recent size and position by this time. The labia majora return to twin studies show that the ability of women to achieve their resting condition in about an hour. About one-third orgasm is partly determined genetically. of women sweat profusely after orgasm, and many have an intense desire to sleep. The Male Sexual Response Cycle Individual Variation The sexual response cycle is quite similar in all males, The experience of orgasm differs among women with individual men differing in the duration more and also at different times in one woman. Figure 8.2 than the intensity of each phase. Also, the physiologi- shows three variations in the female sexual response cal changes in the different phases of the cycle are simi- cycle. In pattern A, a woman goes through a complete lar, regardless of the nature of the stimuli present and cycle, including multiple orgasm. In pattern B, a woman regardless of whether the cycle is initiated by masturba- reaches a plateau and then goes into resolution without tion or by heterosexual or homosexual behavior. Cowper’s gland secretion Full erection Color deepens Prostate enlarges Partially stimulated Scrotum Cowper’s state thickens gland Marked increase in size of testes Testes Unstimulated fully state Partial elevated elevation of testes Excitement Plateau Internal sphincter of bladder closes Seminal vesicles contract Penile contractions Erection Testes disappears descend Unstimulated Urethral state contractions Prostate Loss of force the gland testicular seminal fluid contracts congestion through the urethra Anal Scrotum sphincter thins contracts Orgasm Resolution FIGURE 8.3 The male sexual response cycle. III. PROCREATION  The Sexual Response Cycle 145 Excitement Phase to 50 years old. Masters and Johnson, however, found The excitement phase of the male sexual response the opposite, i.e. older men take longer to gain an erec- cycle (Figure 8.3) can be initiated by any effective erotic tion, but once this is achieved, they maintain an erection stimulus. The first thing that happens is that an erection longer than younger men. begins. The penis stiffens and increases in length and Other physiological changes that occur along with erec- diameter. Thus, the penis is said to become tumescent. It tion in the male excitement phase include the following:    should be noted that an erection can also occur without 1. T he urethral opening (urethral orifice) widens. erotic stimuli being present. For example, it is very com- 2. The scrotal skin becomes congested and thickened, mon for men to gain an erection about every 30–90 min and thus the scrotal diameter is reduced. at night, when rapid eye movement sleep occurs. Also, 3. The testes become elevated due to contraction of many times a man can wake up in the morning with an the cremaster muscle in the scrotum. Stroking the erection. The reason for this “morning erection” is not inner thighs can also cause contraction of this scrotal known, but is probably not caused by a full bladder. muscle. This is the cremasteric reflex. Spontaneous nonsexual erections can also occur if the 4. In about 60% of men, the nipples become more erect. urinary bladder or prostate gland is infected or inflamed, 5. Areas of the skin become reddened due to dilation or they can occur in pubescent males (see Chapter 6). of the blood vessels. This sex flush occurs in about An erection involves a basic biological phenomenon 50–60% of men. that, as mentioned earlier, also occurs in the female sex- 6. The heart rate, blood pressure, and depth and rate of ual response cycle. This phenomenon, vasocongestion, breathing begin to increase. occurs when the flow of blood into a tissue in the arterial 7. There is an increase in tension of voluntary and vessels is greater than the amount of blood that leaves the involuntary muscles (myotonia). tissue in the venous drainage. This results in pooling and    engorgement of blood in the tissue. Erotic stimuli initiate Plateau Phase nerve impulses that travel directly to the spinal cord or to the brain and then to the spinal cord. This initiates an The next phase of the male sexual response cycle, erection reflex by activating an erection center (in the lower given the continued presence of erotic stimuli, is the pla- end of the spinal cord) that contains neurons that control teau phase (Figure 8.3). In this phase, an erection contin- erection. These neurons send their axons to the blood ves- ues and the following changes occur:    sels (arterioles) that supply the erectile tissue in the penis. 1. T here is a slight increase in the size of the glans of Erotic stimuli cause the parasympathetic nerves of the the penis, and its color deepens. The coronal ridge erection center to dominate, and these neurons release (corona glandis) also tends to swell. acetylcholine that causes the arterioles to dilate. This 2. The urethral bulb (enlarged end of the urethra in results in vasocongestion in the blood vessels contained males) enlarges to three times its normal size. in the corpora cavernosa and corpus spongiosum of the 3. There may be preorgasmic emission, from the penis, and the engorgement of blood in these spongy tis- Cowper’s glands, of a few drops of semen. Although sues causes penile tumescence. Recent studies show that slight in volume, this first stage of ejaculation could the neurotransmitter vasoactive intestinal peptide (VIP) is contain some sperm (see Chapter 9). released by parasympathetic nerves along with acetylcho- 4. The testes become more elevated, rotate slightly, line and that intravenous administration of VIP induces and come to lie closer to the groin. Also, the an erection. In turn, VIP may have its effect via other mol- volume of the testes increases by about 50% due to ecules such as cyclic GMP and nitric oxide. Depending on accumulation of fluid. the intensity and effectiveness of the stimuli, an erection 5. The prostate gland enlarges. may be gained partially and then lost repeatedly before a 6. The sex flush, if present, spreads and increases in maximal response occurs. intensity. The sympathetic nervous system, which is dominant 7. There is a further increase in heart rate, blood during stress, also innervates the smooth muscle of the pressure, and the depth and rate of breathing. penile blood vessels. When these neurons are active, 8. There is more tension of voluntary and involuntary they release the neurotransmitter norepinephrine, which muscles.    contracts the penile arterioles, thus reducing blood flow and inhibiting erection. This may be the way that stress Orgasm Phase or fear can inhibit erection. The male now enters the orgasm phase (Figure 8.3): The ability to maintain an erection without ejaculat-    ing seems to vary with age. Kinsey found that males in 1. T  here is a loss of voluntary control of muscles and their late teens or early twenties can hold an erection for a release of neuromuscular tension. There may be up to an hour. This is reduced to 30 min in men from 45 clutching or clawing motions of the hands and feet. III. PROCREATION 146 8. THE HUMAN SEXUAL RESPONSE 2. E jaculation is the expulsion of semen and is controlled causing or maintaining an erection until sexual tension by an ejaculation reflex. There is an ejaculatory center decreases to near resting levels. This refractory period (or spinal nucleus of the bulbocavernosus) in the spinal may last only a few minutes in a young man but may take cord, located higher up than the erection center. more than an hour in an older man. Thus, a younger man When activated, this center sends sympathetic may be able to have several orgasms, each separated by neural stimulation to the bulbocavernosus muscle a few minutes. The amount of semen, however, is less in at the base of the penis. Ejaculation occurs in later ejaculations. The duration of the refractory period two phases. First there is a specific sequence of in young men is about 10 min but can be influenced by contraction of smooth muscle in the walls of the fatigue and amount of sexual stimulation. Kinsey found testes, epididymides, vas deferens, ejaculatory duct, that 6–8% of the men he studied had more than one seminal vesicles, prostate gland, bulbourethral orgasm during one sexual encounter, and these men glands, and urethra. These contractions expel semen reported that the initial orgasm was the most pleasurable. into the urethral bulb. Simultaneously, a muscular There is a misconception that many older men die of sphincter that guards the opening of the urethra into a heart attack during coitus. Actually, the risk of having the urinary bladder contracts, thus preventing urine a heart attack in a man within any 2 h is 10 in 1 million, from entering the urethra and semen from entering and this risk is 20 in 1 million within 2 h of beginning sex. the bladder. This series of events constitutes the emission stage of ejaculation and is experienced by a Resolution Phase male as a sensation of imminent ejaculation. Smooth During and after the refractory period (and if no effec- muscle contractions of the male reproductive tract tive erotic stimuli are present), the male goes through may be influenced by the hormone oxytocin and the resolution phase, in which the arousal mechanisms by the presence of prostaglandins in the seminal return to a resting state (Figure 8.3). In this phase, the fluid. The expulsion stage of ejaculation begins erection is lost because the erection center is now domi- next, with rhythmic contractions of the penis and nated by the activity of sympathetic neurons. This bulbocavernosus muscle, which lies at the base of the causes the arterioles supplying the penile spongy tissue penis. The first three or four of these contractions to constrict, thus reducing vasocongestion. About 50% are intense and result in a forceful expulsion of penis size is lost rapidly. Other responses that occur of the majority of the semen from the urethra. rapidly include disappearance of the muscle tension and The contractions that follow are less intense and sex flush and a lowering of heart rate, blood pressure, produce smaller volumes of semen. These expulsion and respiratory rate (all usually in about 5 min). Other contractions are approximately 0.8 s apart. changes taking a longer time include final reduction in 3. The testes are at their maximal elevation. penis size, relaxation of the scrotum, descent of the tes- 4. The heart rate peaks as high as 180 beats per minute tes, and loss of nipple erection. About one-third of men (from a resting rate of about 70). The blood pressure sweat over their body, and many experience an intense peaks at about 200 over 110 (from a resting pressure desire to sleep. The entire resolution phase can take up to of about 130 over 70). The respiratory rate peaks 2 h. Close physical contact with the partner, such as keep- at about 41 breaths a minute (from a resting rate of ing the penis within the vagina, touching, and caressing, about 12 per minute). can delay male resolution. A desire or attempt to urinate 5. The sex flush, if present, peaks in intensity and can speed up resolution. distribution.    Why Did Orgasm Evolve? For most men, ejaculation is an essential component of the experience of orgasm. Orgasm, however, can occur To humans, orgasm is an intensely pleasurable expe- without ejaculation. For example, during retrograde rience, but is it directly necessary for reproduction? The ejaculation, the emission stage occurs but not the expul- answer is no. As discussed in Chapter 9, female orgasm sion stage, resulting in a “dry orgasm” when the semen is not necessary for fertilization to occur, and some men enters the urinary bladder. This can be due to physical can ejaculate without having an orgasm. damage to the urethra or to a relaxed urinary sphincter One theory about the evolution and adaptive value muscle. Also, coitus reservatus (see Chapter 13) has been of orgasm is as follows. Most men experience orgasm practiced as a birth control method by some people, e.g. when they ejaculate, whereas fewer than half of Ameri- in India. In this method, men learn to approach ejacula- can women experience orgasm each time they have sex. tion repeatedly with no expulsion. A vast majority of women do not have orgasm unless Immediately after ejaculation, the male (unlike the they receive effective clitoral stimulation, and one idea female) enters a refractory period (Figure 8.2). During is that only a man who is caring, knowledgeable, and this period, potentially erotic stimuli are not effective in sensitive can assist his partner in orgasm. The orgasmic III. PROCREATION  Hormones and Sexual Behavior 147 response in the woman would then be a reward to the deriving sexual pleasure from self-stimulation of the man; i.e. it would make sex more pleasurable for him. genitals. Thus, a pair bond based on caring, sensitivity, and plea- In anal coitus, the penis penetrates the anus and is sure is mediated at least partially by female orgasm. moved within the rectum. This method of coitus is com- Female orgasm then may have evolved as a mechanism mon in male homosexuals and in some heterosexual of mate choice, ensuring that a woman’s long-term couples. A heterosexual couple should use a condom and partner is sensitive to her needs (sexual and otherwise) never switch from anal to vaginal coitus before washing and thus more likely to be a good provider for their the penis, as the rectum contains microorganisms that offspring. Other theories of female orgasm include the could infect the female reproductive tract (see Chapter idea that muscular contractions during orgasm may 17). The walls of the rectum are not as well lubricated help draw sperm into the uterus. Finally, the sense of as are those of the vagina, and the anal sphincter is con- relaxation and sleepiness often produced by orgasm stricted. Therefore, lubrication of the anus and penis may promote sperm retention by the female reproduc- with saliva or a sterile lubricant is common. tive tract. Oral coitus is contact of the mouth with the genital organs. When the mouth of the partner touches the geni- tals of a female, it is called cunnilingus (Latin cunnus, COITUS (SEXUAL INTERCOURSE) meaning “vulva”; lingere, meaning “to lick”). Cunnilin- gus is practiced in several cultures. One danger of this Coitus (Latin coitio, meaning “a coming together”) is, form of oral coitus is the possibility of air being blown for many of us, a vehicle for the expression of emotion into the vagina, as air bubbles could enter the blood- and intimacy. Strictly speaking, coitus (or sexual inter- stream and could be dangerous. Therefore, air should course) is the penetration of the vagina by the penis, not be blown into the vagina. which can be called vaginal coitus (Figure 8.4). However, Fellatio (Latin fellare, meaning “to suck”) is the oral the term coitus is also used for other forms of sexual con- manipulation of the penis or scrotum by a sexual part- tact, including oral coitus (oral–genital contact), femoral ner. Some worry about the adverse effects of swallowing coitus (when the penis is inserted between the thighs), the semen, as it can contain microorganisms such as HIV mammary coitus (when the penis is inserted between (see Chapter 18). Obviously, a woman cannot get preg- the breasts), and anal coitus (insertion of the penis into nant from this form of coitus. the rectum). There are many common slang phrases for coitus, such as “making love,” “going to bed,” and other more descriptive phrases. Legally, fornication is the vol- HORMONES AND SEXUAL BEHAVIOR untary coitus between an adult man and woman who are unmarried. Adultery is voluntary coitus between two In many animals, certain hormones need to be pres- people, at least one of whom is married to someone else. ent for sexual behavior to be expressed. There are essen- Sodomy means different things in different states; it usu- tially two ways that a hormone can stimulate (or inhibit) ally refers to anal or oral coitus, but also can mean “acts sexual behavior. First, the hormone can act directly on against nature” such as coitus with an animal. Finally, the brain to affect sensitivity or activity of neurons that masturbation, which is not a form of coitus, is the act of influence sex drive, or libido. This is called a central effect Seminal vesicle Urinary bladder Male pubic bone Prostate gland Bulbourethral gland Clitoris Urethra Female pubic bone Vas deferens Penis Urinary bladder Epididymis Uterus Testis Cervix Vaginal Vaginal opening cavity Anus FIGURE 8.4 Representation of the erect penis inserted into the vagina during vaginal coitus. III. PROCREATION 148 8. THE HUMAN SEXUAL RESPONSE of a hormone. Second, a hormone can affect the sensitiv- higher the testosterone levels in the blood, the less time ity or growth of peripheral tissues, such as skin or mus- it takes to achieve maximal penile erection. If a man is cle, that are involved in sexual behavior. This is called a given excess androgen, it is not known to produce an peripheral effect of a hormone. In both instances, however, excessive sex drive. proper erotic stimuli are necessary for the behavior to Many factors can influence the levels of testosterone be expressed, with the hormones simply increasing or in a man’s blood and, therefore, potentially his sex drive. decreasing sensitivity and/or response to these stimuli. First, there is a daily rhythm of testosterone secretion, The region of the human brain that influences sexual peaking in the morning (see Chapter 4). Also, anticipa- behavior is in the limbic system, which includes the thal- tion or performance of sex may increase testosterone amus, amygdala, hippocampus, and part of the hypo- secretion. Erotic arousal stimulates LH secretion in men, thalamus and cerebral cortex (see Chapter 16). In fact, which may raise testosterone levels. In Rhesus mon- when electrical stimulation is applied to the human lim- keys, dominant males have relatively high testosterone bic system during brain surgery, a sexual response can levels, which decrease when they become subordinate. be elicited. We now discuss the role of hormones in the This may also be true, to a modified extent, in humans. human sex drive and the sexual response. Finally, there is a tendency for blood testosterone levels to decrease with age. Sexually active 70-year-old men have higher testosterone levels than sexually inactive Hormones and Male Sexual Behavior men of the same age. We discussed the subject of “andro- In other mammals, the androgen testosterone stimu- pause” in Chapter 7. lates the male sex drive. If adults of these animals are orchidectomized (the testes are removed), the sex drive Hormones and Female Sexual Behavior disappears. That is, the males show no behavioral response to stimuli from a female. Administration of tes- In females of many animal species, it is an estrogen tosterone can restore sex drive in these orchidectomized (sometimes acting with progesterone) that increases the males. This androgen has a central effect on the brain sex drive. In human females, however, the story appears and also increases the sensitivity of the penis to tactile to be more complicated. If an adult woman has her ova- stimuli. It has been discovered that testosterone does not ries removed (ovariectomy), this generally has little or no directly affect male sex drive in the brain of laboratory influence on her libido, although her breasts, vagina, and mammals. Instead, testosterone is converted enzymati- uterus will shrink because of the absence of estrogen. If, cally to estradiol by cells in part of the limbic system, however, her adrenal glands are removed along with her and it is this estrogen that actually increases male sex ovaries, her sex drive decreases dramatically. Then, if this drive. Whether or not this is true in humans is not clearly woman is given an androgen, her sex drive is restored. known (Chapter 16). Remember from previous chapters that the adrenal Testosterone also influences the sex drive in human glands secrete “weak androgens” (predominantly dehy- males, but this influence is affected markedly by learn- droepiandrosterone [DHEA]) and that the ovaries also ing. Orchidectomy of human males before puberty gen- secrete testosterone, with a peak in blood levels of this erally decreases the development of male sex drive at androgen around ovulation. It is now generally thought puberty. In ancient China and Egypt, some young males that in women, the weak androgens secreted by the adre- were orchidectomized. They not only failed to develop nal glands, perhaps in concert with ovarian estradiol and male secondary sexual characteristics but also were ovarian testosterone, control the sex drive. Secretion of reported to have little or no sex drive. These men, called adrenal androgens in women is greatest from puberty eunuchs (Greek for “guardian of the bed”), were then through the late twenties, declines between ages 30 and trusted to guard females in royal harems. 50, and then remains at steady low levels in later years. If a man is orchidectomized after he has reached Other hormones also seem to affect the female sex puberty, the usual results are a slowly declining sex drive. Progesterone, for example, tends to lower female drive and a gradual loss of the ability to have an erec- libido, and combination birth control pills with high tion. The extent of the loss of libido varies from one man amounts of a progestin may also have this effect. In addi- to the next, emphasizing the role of learning in male tion, gonadotropin-releasing hormone (GnRH) directly sexual motivation and that androgens may not be as stimulates female sexual behavior in some mammals. necessary after the original behavior pattern develops. We have much to learn about the role of GnRH in human Usually, penis size and the extent of facial and body hair sexual behavior, but it is possible that GnRH stimulatory are not influenced by orchidectomy. Administration of agonists can eventually be used to increase human sex an androgen to an orchidectomized man who suffers a drive in patients with low libido. decrease in sex drive can restore his libido and the abil- If hormones influence female libido, there could be a ity to have and maintain an erection. In normal men, the variation in sex drive during the menstrual cycle. The III. PROCREATION  Hormones and Sexual Behavior 149 evidence for this possibility, however, is controversial. about 1.3% of adult females had homosexual sex in the For example, some studies have used frequency of coitus past year and 3.8% since the age of 18. Among Ameri- during the menstrual cycle as an indicator of sex drive. can males, 2.7% had gay sex in the previous year and It is also clear that coitus is influenced by many psycho- 7.1% since age 18. Slightly higher percentages reported logical factors, such as opportunity, fear of pregnancy, being attracted to the same sex. It should be noted that and fatigue. Kinsey, as well as Masters and Johnson, a person may, for various reasons, never have expressed found an increase in frequency of coitus during the three homosexual tendencies through behaviors, even though or four days before menstruation begins, which may his or her feelings are almost exclusively homosexual. It reflect a desire to avoid conception. However, studies should also be pointed out that there are many hetero- also show an increase in coital frequency just after men- sexuals who have homosexual thoughts or fantasies at struation and near ovulation. The general conclusion one time or another in their lives but have no tendencies seems to be that, like other mammals that exhibit a peak to express these in their sexual behavior. Finally, there in sex drive (estrous behavior) around the time of ovula- can be situational homosexual behavior. In prisons, for tion (see Chapter 3), human females have retained some example, homosexual behavior is more common than in peak near ovulation. However, the actual expression of the general population. This is not because more homo- this increase in sex drive can be influenced greatly by sexuals commit crimes and therefore end up behind psychological factors. Therefore, unlike in animals with bars. Rather, much of this behavior consists of hetero- a distinct behavioral estrous cycle, the human female can sexuals switching to homosexual behavior because there be sexually receptive at any time during the menstrual is no heterosexual outlet for their sex drive. Thus, the cycle. fact that homosexual rape is common in prisons can be explained by the need to establish dominant and subor- dinate relationships in the prison. You can see from this Homosexuality discussion that it may be difficult to classify a person as Homosexual behavior is sexual contact with a member of a “homosexual,” and sometimes it is more appropriate the same sex. Persons who frequently or always choose to talk about homosexual behavior. a member of the same sex in their sexual relations, or The sexual behaviors of homosexuals are similar usually are sexually attracted to the same sex, are called to those of heterosexuals except for, of course, vaginal homosexuals (Greek homo meaning “same”). Male homo- coitus. Kissing, hugging, and caressing can be included sexuals are referred to as gay. Female homosexuals often in foreplay of homosexuals, as can fellatio and anal prefer the term lesbian, a name derived from a Greek coitus in homosexual males, cunnilingus in lesbians, and homosexual poetess named Sappho, who lived on the mutual masturbation in both sexes. A study by Masters island of Lesbos (now Mytilene) in the Aegean Sea in and Johnson indicates that the sexual response cycles 600 B.C. In contrast, a heterosexual person is attracted of homosexuals and heterosexuals are physiologically to and chooses to have sexual relations with members identical regardless of the type of stimuli present. of the opposite sex (heterosexual behavior.) One’s sexual What factors predispose an individual to homosex- orientation refers to the gender to which one is sexually ual behavior as an adult? The answer is that we are not attracted. sure, but there are several theories. Some psychological Kinsey contributed greatly to our understanding of theories suggest that, during childhood and/or ado- human sexual behavior in pointing out that there is a lescence, sexual orientation is formed through family continuum from heterosexuality to homosexuality. In his dynamics or interactions with peers or adults. However, scale (Figure 8.5), a person with a score of 0 is exclusively much evidence suggests that there is a biological basis heterosexual. At the other extreme are exclusively homo- of homosexuality. For example, sometimes children are sexual individuals, receiving a score of 6. These people born with ambiguous genitalia; i.e. their sex cannot be always choose a sex partner of the same sex. Using this determined with certainty at birth. Genetic males born scale, about 2% of male Americans and 1% of female with underdeveloped external genitalia are often raised Americans have a score of 6 and are exclusively homo- as girls. One study of these individuals found that, as sexual. About 63% of males and 87% of females have a adults, the majority were attracted to women despite score of 0 and are exclusively heterosexual. The remain- having been treated as girls their whole lives. Several ing people, 35% of males and 12% of females, are bisexual studies have shown that homosexual and transgendered in that they have had sexual experiences with the same men and women often display evidence of an altered and the opposite sex at one time or another in their adult gender identity very early in life. These so-called gen- life. der nonconforming children prefer the clothing, toys, Surveys more recent than Kinsey’s show that these and activities associated with the opposite sex. Much frequencies of homosexual, heterosexual, and bisexual current research focuses on the effects of hormones dur- behaviors remain similar today. In the United States, ing prenatal development. Because early exposure to III. PROCREATION 150 8. THE HUMAN SEXUAL RESPONSE Kinsey scale 6 5 4 3 2 1 0 0 1 2 3 4 5 6 Kinsey scale FIGURE 8.5 The Kinsey adult heterosexual-homosexual continuum: 0, exclusively heterosexual behavior; 1, mostly heterosexual, with inci- dental homosexual experience; 2, heterosexual, with substantial homosexual experience; 3, an individual with similar amounts of heterosexual and homosexual experience; 4, homosexual, with substantial heterosexual experience; 5, mostly homosexual, with incidental heterosexual expe- rience; and 6, exclusively homosexual behavior. The proportion of each arrow shaded gray represents the percentage of heterosexual behavior. Individuals with scores of 1 through 5 could be termed bisexual. testosterone masculinizes the brain of male mammals, homosexuality in women has also been shown in twin scientists theorize that homosexuals may have been studies of lesbians. These reports have been criticized exposed to abnormal levels of hormones as fetuses. Sev- because they used volunteer participants, and it was eral lines of evidence support this theory, including the suggested that more participation by gay twins could discovery of feminized areas in the brains of some gay influence the results. To eliminate this possible effect, the men. We shall discuss these issues in Chapter 16. largest twin study so far collected data from a registry of There may also be an inherited component to homo- all fraternal and identical adult twins in Sweden. Of these sexuality; it tends to run in families. One kind of evi- adults, 5.6% of men and 7.8% of women reported having dence in support of a genetic basis of homosexuality same-sex sexual partners. The study found that genetic comes from studies of identical and fraternal twins. For factors accounted for 34–37% of homosexual behavior in example, a 1991 study found that in males, if one twin is men and 18–19% in women. The shared family environ- homosexual, there is a 52% chance that his identical twin ment accounted for 0% of homosexual behavior in men will be too, a 22% chance that his fraternal twin is gay, and and 16% in women. Environmental effects unique to the only an 11% chance that his adoptive brother is homo- individual (which could include biological factors such sexual, being raised in the same household. Even though as prenatal hormone exposure and childhood illness, or the 11% incidence for adopted brothers is the lowest, it psychological influences such as peer group interactions is still higher than the percentage of homosexual men in and sexual experiences) explained about 65%. These the general population. A significant genetic influence on studies therefore show a substantial genetic influence on III. PROCREATION  Hormones and Sexual Behavior 151 both male and female homosexuality, with the environ- insemination (for lesbian couples), or were offspring of ment also playing an important role. previous heterosexual sex. In 1993, a small region (Xq28) on the X chromosome If there is an inherited component to homosexuality, was identified as a “gay gene” in males; it actually con- why have the genes related to homosexuality not disap- tains about 200 genes. First, it was discovered that only peared? After all, homosexuals usually do not father or brothers, maternal uncles, and maternal male cousins bear children. Maybe it confers evolutionary fitness in exhibited an increased incidence of homosexuality in another way. For example, a recent study shows that families of gay men. Then it was discovered that the X female relatives of gay men have more children. Perhaps chromosomes of 33 out of 40 pairs of fraternal broth- homosexuals in our ancestors’ times were parenting ers who were both gay shared this gene cluster on their “helpers,” and thus gay gene(s) were retained because X chromosome. Only 25% of 12 heterosexual brothers these individuals helped raise their relatives. One the- shared this genetic cluster. This gene area is on the end of ory posits that male homosexuality is a polygenic trait the long arm of the X chromosome and is inherited from involving several genes that feminize brain develop- the mother. Finally, a similar study of 36 pairs of lesbian ment. Heterosexual carriers of these alleles would tend sisters lacked this gene cluster. The authors concluded to be more empathetic and sensitive, traits that make that an inherited factor coded for on the maternal X chro- for more attractive mates and better fathers. This would mosome is related to male homosexuality, but it is not keep the alleles in the gene pool. the only determinant of male homosexuality as seven In summary, we do not know what causes homo- of the gay brothers did not share this region. Other, but sexual behavior or, for that matter, heterosexual behav- not all, subsequent studies confirmed linkage between ior. All human behavior is a complex interaction among Xq28 and male homosexuality, so this continues to be inheritance, development, physiology, and learning expe- an area of investigation. Female homosexuality may be riences, and it is difficult to determine the relative influ- controlled by different factors than male homosexuality. ences of these factors. Furthermore, scientists who attempt There are indeed other observations that homosexual- to determine the causes of adult homosexual behavior ity, like heterosexuality, has a genetic, early developmen- often come up with differing conclusions. For example, tal basis. For example, if one numbers the fingers on one adult homosexuals do not always exhibit reversed gen- hand from I (the thumb) to V (the little finger), the length der identity. However, a study by the Kinsey Institute ratio of fingers II and IV is informative. In heterosexual of Sex Research suggests that a disturbance of gender men, on average, II is shorter than IV, but in homosexual identity in childhood is very common in both male and men, II is equal to IV. In heterosexual women, the two female homosexuals. That is, male homosexuals exhibit fingers are of equal length, but in lesbian women, II is feminine behaviors as young boys, whereas the reverse shorter than IV on average. Differences in the growth of is true for female homosexuals. This study also suggests finger bones are thought to reflect androgen levels dur- that homosexual behavior has its origins not as a result of ing development; whether there is a direct genetic basis a disturbed family environment but as a deep-seated pre- is not known. Another indication that homosexuality has disposition, perhaps biological in origin. Thus, perhaps an early, neurodevelopmental basis is that a homosexual there are several reasons why an adult may exhibit homo- is 39% more likely to be left-handed than a heterosexual; sexual behavior, and we should speak of different kinds this relationship between handedness and homosexuality of homosexuality. is stronger in women than in men. Male homosexuals also In 1973 the American Psychiatric Association removed have more older brothers than heterosexual men; each homosexuality from its list of “personality disorders and older brother increases the chance of homosexuality in a certain other nonpsychotic mental disorders.” Clinicians younger brother by 33%. The proposed explanation is that now agree that homosexuality is not an illness; it is just a a pregnant woman’s immune system raises antibodies sexual orientation. In recent decades, homosexuals have against molecules on the surface of cells in her developing become freer to express their sexual preference in public, son’s brain. During a later pregnancy, the antibodies could a choice often called “coming out of the closet.” Many alter development in male brain regions associated with homosexuals feel better about themselves